The construction of so called conventional ion-selective electrodes (ISE) employing different ion-selective membranes can be depicted by the following scheme:
Examined solution .vertline. Ion-selective membrane .vertline. Reference solution .vertline. Reference element
where the vertical bar represents an interface between two phases. In this conventional type of ISE the ion-selective membrane is in electrical contact with the reference solution, depending on the ion-exchange equilibrium. The reference element is in electronic contact with the external electronic instrumentation. The transfer from ionic conductivity (in the membrane and the reference solution) to electronic conductivity (in the reference element and external instrumentation) is provided by the reversible electrode reaction of the reference element. By a proper choice of the reference element and the composition of the reference solution it is possible to obtain an ISE with a stable and reproducible standard potential [1].
In some applications it is advantageous to replace the reference solution by a solid contact giving an all-solid-state ISE [1]. One such approach is to attach the ion-selective membrane directly to a solid, electronically conductive substrate as represented by the following scheme [2]:
Examined solution .vertline. Ion-selective membrane .vertline. Substrate
This type of ISE, hereinafter referred to as the coated-wire electrode (CWE), can be fabricated in various shapes and sizes. It can be miniaturized, and it may be inexpensive to produce since the substrate does not necessarily have to be a noble metal. However, irreproducibility and drift of the electrode potential are a problem usually encountered with the CWE. The instability of the potential may be caused by the blocked interface between the ionically conducting ion-selective membrane and the electronically conducting substrate [3].
Several approaches to improve the potential stability of the CWE have been tried [1]. One such approach is to connect the ion-selective membrane to the solid substrate via an intermediate layer having mixed ionic-electronic conductivity, as represented by the following scheme:
Examined solution .vertline. Ion-selective membrane .vertline. Intermediate layer .vertline. Substrate
In this type of ISE, hereinafter referred to as the solid-contact ISE (SCISE), the transfer from ionic to electronic conductivity is possible due to the mixed ionic-electronic conductivity of the intermediate layer. The ion-selective membrane is in ionic equilibrium with the intermediate layer, which is in electronic equilibrium with the substrate [1]. Doped conjugated polymers like p-type polypyrrole can be regarded as mixed ionic-electronic conductors an they can be used as an intermediate layer in the SCISE [4].
The main advantage of using an intermediate layer between the ion-selective membrane and the substrate is to improve the stability of the electrode potential, i.e. SCISEs are more stable than CWEs. However, due to the need for an intermediate layer, SCISEs are more complicated to fabricate than CWEs.
Another approach to improve the potential stability of the CWE is to incorporate an oxidable and reducible substance, a so-called redox agent in the ion-selective membrane, as represented by the following scheme[1]:
Examined solution .vertline. Ion-selective membrane .vertline. Redox agent .vertline. Substrate
In this type of ISE, hereinafter referred to as the redox-loaded electrode (RLE), electronic equilibrium exists between the redox agent in the membrane and the electronically conducting substrate, resulting in improved stability of the electrode potential as compared to the CWE. In some cases, however, the RLE shows potential drift due to dissolution of the redox agent in the membrane [1]. In other cases the life time of the electrode is short due to disintegration of the membrane phase. Depending on the amount and quality of the redox agent in the membrane, there is also the risk that the RLE will be redox sensitive due to the possibility of electron transfer at the membrane-solution interface.
In the present invention, an electronically conducting or semiconducting conjugated polymer is mixed with an ion-selective membrane, resulting in a novel type of single-piece all-solid-state ion-selective electrode (SPE).